Export volume name to tikv metrics

[polyrabbit]

A TiKV cluster can serve metadata for multiple JuiceFS volumes. This PR exposes each JuiceFS volume name as a tenant identity in TiKV request-source metrics, which allows operators to monitor:

• Request rate per volume with tikv_grpc_request_source_counter_vec
• Average TiKV request latency per volume with tikv_grpc_request_source_duration_vec/tikv_grpc_request_source_counter_vec

When a TiKV cluster is under pressure, this makes it easier to quickly identify which volumes contribute most of the request traffic.

Following dashboard screenshot shows real per-volume source labels after this change:

[codecov]

Codecov Report

❌ Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 22.00%. Comparing base (75730ca) to head (23e2593).
⚠️ Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/meta/tkv_tikv.go	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7480      +/-   ##
==========================================
+ Coverage   21.95%   22.00%   +0.05%     
==========================================
  Files          31       31              
  Lines       24747    24805      +58     
==========================================
+ Hits         5433     5459      +26     
- Misses      18719    18749      +30     
- Partials      595      597       +2     

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